Ruthenium-Catalyzed Hydration of 1-Alkynes to Give Aldehydes: Insight into anti-Markovnikov Regiochemistry

Abstract
The mechanism of the selective conversion of 1-alkynes to aldehydes by hydration was investigated by isolating organic and organometallic byproducts, deuterium-labeling experiments, and DFT calculations. The D-labeled acetylenic hydrogen of 1-alkyne was found exclusively in the formyl group of the resulting aldehydes. After the reaction, the presence of metal-coordinated CO was confirmed. All of the experimental results strongly suggest the involvement of a metal−acyl intermediate with the original acetylenic hydrogen also bound to the metal center as a hydride, with the next step being release of aldehyde by reductive elimination. Theoretical analyses suggest that the first step of the catalytic cycle is not oxidative addition of acetylene C−H or tautomerization of η2-alkyne to a vinylidene complex, but rather protonation of the coordinated 1-alkyne at the substituted carbon to form a metal−vinyl intermediate. This cationic intermediate then isomerizes to Ru(IV)−hydride−vinylidene via α-hydride migration of the vinyl group to the metal center, followed by attack of the vinylidene α-carbon by OH- to give the metal−hydride−acyl intermediate.

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